It is often claimed: "History is written by the victors". This oft-quoted aphorism about history and war could equally be applied to the arts, sciences, and many other disciplines where the victors in these fields are 'men' and the vanquished 'women'. I am no longer surprised--sadly so--when I find yet another woman whose accomplishments have been buried and credit for those accomplishments either taken by men or never mentioned at all. Fortunately, it seems, this trend seems to be changing in my lifetime. Still, it never ceases to amaze how our own understanding of so many things is narrowed by the victor's narrative. Sometimes it takes some effort to find out the whole story...

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This image, of course, is the famous M31 Andromeda Galaxy. Iconic. We all know what it looks like and may have multiple versions in our portfolios. I myself photographed it one year ago in homage to Hubble's analysis of the V1 variable. That image is here.

What is perhaps not known, however, is that the important discovery required for Hubble's analysis was done by Henrietta Swan Leavitt (1868-1921). In 1908 Leavitt published a paper entitled "1777 Variables in the Magellanic Clouds". These variables were what we now call Cepheid variables and Leavitt concluded "It is worthy of notice that the brighter variables have the longer periods". She knew that there was something special going on and that more research was needed. This would become the focus of her professional life over the next decade.

Leavitt and a group of other highly-trained, mostly now-forgotten women worked under Edward Pickering (of "Pickering's Triangle" fame) at Harvard College Observatory. They were referred to as 'Computers' and the women collectively and derisively referred to as "Pickering's Harem". Although plagued by illness, increasing blindness and deafness, Leavitt continued to work on Cepheid variables, meticulously examining photographic plates while working on other assigned tasks. In 1912 she made her breakthrough analysis, discovering that there is a precise relationship between the distance and brightness of this class of stars (inverse square law). Leavitt published the results in 1912, but credit largely went to Pickering, who put his own name on the paper alongside Leavitt's. 

Thanks to Leavitt, we now had another way to measure both stellar and intergalactic distances--the "Standard Candle"--a term coined by Leavitt. It was this discovery that paved the way for Ejnar Hertzsprung to measure distances to nearby Cepheids (using parallax) and allowed Hubble to measure the distance to V1 in Andromeda and then later on to conclude that the universe was expanding. 

While Leavitt may have been called a "computer" by her male counterparts, in the 1920 US Federal Census, Leavitt was quite clear when she self-reported her own occupation: Astronomer.

I couldn't agree more! While I don't for a moment believe that my image of Andromeda amounts to anything at all, I post it as a timely excuse to pay homage to Henrietta Leavitt for all of her meticulous work and keen observations that opened the door to understanding just how large our universe really is!

See additional information provided by @John Favalessa in the comments below.

Sources:
Henrietta Leavitt--Celebrating the Forgotten Astronomer. By Gael Mariani, AAVSO.
Henrietta Swan Leavitt. Famous Scientists: The Art of Genius.